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Help me to understand FL7760 IC

Rickooo

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Hi, I want to convert 12V to 9V 2.5A constant current.
I am using FL7760 IC for the same.
Please find the attached circuit.
When I test it on PCB and gave 12V input to the circuit from power supply. I got same 12V at output without load when I measure using multimeter. It's does not buck. Can you please guide me, why? Also, I have just add capacitor and ground it to the pin Vcc (as I found in datasheet). I didnt gave 5V supply.
1603541343377.png
 

Carry for cents bazar

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Is this a voltage buck or current buck circuit? What happens when you connect the load?
--- Updated ---

I see that the control of this circuit depends on the voltage sensing over the load.
 

KlausST

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Hi,

it is a constant current ciruit, it doesn´t care about output voltage.
Output voltage depends on load (resistance) --> your LEDs.
Without load it´s quite expectable to go to 12V.

Why are you expecting 9V?
Where and why do you expect 5V?

Klaus
 

Carry for cents bazar

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Klaus is making it too easy. The current on the output will not change, but the voltage will depending on the load. Also you are not using the datasheet circuit.
 

Rickooo

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Thank @KlausST and @Carry for cents bazar
I tested it on PCB without load. As LED is of 9V and output I am getting is 12V. SO, I am unsure if I can connect 9V led to 12V output. Also, if I add load resistance, how much it suppose to be. How will I calculate? WIll it be 30W(12V 2.5A(consider)) - 22.5W(9V 2.5A)?
Can I test buck converter without load?
 

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You can test the converter current regulation by connecting a low ohmic resistor or even a short as load.
 

Rickooo

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Hi,

Give ohm´s law a try:
What load voltage do you expect?
What load current do you expect?

Klaus
Volt is 9V
Current is 2.5A.
According ohm's law: it is 3.6 ohm. If yes, what will be the wattage of this resistor?
 

Rickooo

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Hi, I am using FL7760 ic to drive the led. I made a buck circuit. In which I give 12V input and want 9V 750mA output to drive led. But, unfortunately, I am getting 0V with no load. And, the supply is also not supply above than 7V at the input side. Why? Will this ic support 750mA current? Please help me if you find something in the datasheet. Thank
 

Rickooo

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Rather contradicting statements. What happened?

Klaus
Yes, Sometime, led works and get required volt. And sometime, I get 0V. In datasheet, in few circuit they gave 5V Vcc and in some, they just left it to ground. I my circuit, I just add capacitor. Is it because of this? I need to give vcc? If yes, Why and how much supply?
 

KlausST

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Hi,

Your descriptions are very vague.

If you refer to a datasheet then give a link to the datasheet.
If you talk about a circuit then tell which circuit you are talking about.
If you talk about a capactitor then tell which capacitor you are talking about.
If you talk about any tests tell us the test conditions.

We want to help, but we can´t. It´s just confusing and incomplete informations.

Klaus
 

Rickooo

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Hi,

Your descriptions are very vague.

If you refer to a datasheet then give a link to the datasheet.
If you talk about a circuit then tell which circuit you are talking about.
If you talk about a capactitor then tell which capacitor you are talking about.
If you talk about any tests tell us the test conditions.

We want to help, but we can´t. It´s just confusing and incomplete informations.

Klaus
Datasheet: https://www.onsemi.com/pub/Collateral/FL7760-D.PDF also, please read post #1 and look to the attached circuit. At VSS, I just ground it, and didn't gave 5V.
 

KlausST

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Hi,

please read post #1 and look to the attached circuit. At VSS, I just ground it, and didn't gave 5V
Ok. I´ve read it.
* Where can I find VSS? In the schematic? In the datasheet?
* What did you ground?
* What did not gave 5V?
* What does "5Vin" in your schematic mean? Did you additionally connect something or not?

Klaus
 

BradtheRad

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Does your IC need a 5V power supply? Then it may have trouble taking readings on a sense resistor which is fed by 12V.

Your converter appears to be referenced to the positive supply (rather than to ground). It can work if your IC is adjusted to create 75 percent duty cycle to the inductor based on readings across your sense resistor.

Here's my simulation showing expected waveforms. Operating frequency with a 6uH inductor is several hundred kHz. Notice led receives a wide range of current above and below 750mA. Yet voltage is 9V with slight variation.

buck conv ref'd to posi supply clk-driv-swi 12v to 9v led 900mA.png
 

Rickooo

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Hi, I am working on making 4 LED driver circuit, where each LED is 3V and they are attached in series.
1. CIRCUIT 1: 12V 9V 2.5A
2. CIRCUIT 2: 12V to 9V 1.5A
3. CIRCUIT 3: 12V to 9V 1A
4. CIRCUIT 4: 12V to 9V 750mA

Problem I got:
When I test the circuit with no load:
CIRCUIT 1 & CIRCUIT 2 gave me Vo = Vin. And CIRCUIT 3 & CIRCUIT 4 gave me Vo = 0V. Why?
Note: Value of each element is calculated by excel file available on ONSemi website. Also, please find the attached excel here too.

The IC datasheet: https://www.onsemi.com/pub/Collateral/FL7760-D.PDF
Circuit I am designing: 1604058467408.png
 

FvM

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1 + 2 is expectable, as already explained along this thread. 3 + 4 is not expectable, there's presumedly a wiring error or defective part.
 
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